Masquerade host to flannel traffic.

Traffic coming from host might have src address not of
flannel interface but of some other one. This happened
in conjunction with REDIRECT rules. Masquerade will
force the src addr to be that of the flannel interface.

Fixes #318

network/ipmasq.go

@@ -40,8 +40,10 @@ func setupIPMasq(ipn ip.IP4Net) error {
 		{"FLANNEL", "-d", ipn.String(), "-j", "ACCEPT"},
 		// NAT if it's not multicast traffic
 		{"FLANNEL", "!", "-d", "224.0.0.0/4", "-j", "MASQUERADE"},
-		// This rule will take everything coming from overlay and sent it to FLANNEL chain
+		// This rule will take everything coming from overlay and send it to FLANNEL chain
 		{"POSTROUTING", "-s", ipn.String(), "-j", "FLANNEL"},
+		// Masquerade anything headed towards flannel from the host
+		{"POSTROUTING", "!", "-s", ipn.String(), "-d", ipn.String(), "-j", "MASQUERADE"},
 	}
 
 	for _, rule := range rules {